Upon outputting grayscale image data to an output device, density transformation is popularly done to improve image quality. For example, JP 3,163,753 (B2) discloses a technique for quickly inspecting the density distribution by inspecting that of low-frequency components in frequency data obtained by transforming grayscale image data into spatial frequency components, and transforming the density on the basis of the inspection result.
According to the above technique, the density of an image can be transformed. However, density transformation corresponds to linear gamma correction, and cannot correct color fog and the like of a photo image.
A photo-direct printer as a non-PC device normally has a RAM size of several Mbytes in terms of cost limitations and the like, and its storage capacity is as small as around 1/16 to 1/64 of those of recent PCs. The photo-direct printer must often store a program, and data associated with control of the printer and interface in a single RAM, thus imposing severer memory limitations. In recent years, since the number of pixels of a digital camera increases, the data size per photo increases, and some data exceed 1 MB per photo.
In consideration of the above circumstances, when a work memory has an insufficient storage capacity, access to an external storage device (e.g., a memory card) occurs frequently, resulting in a considerable drop of the processing speed.